In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plates, also called printing plate precursors, typically comprise an imageable layer applied over the hydrophilic surface of a substrate. The imageable layer typically comprises one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. If, after imaging, the imaged regions of the imageable layer are removed in the developing process revealing the underlying hydrophilic surface of the substrate, the precursor is positive-working. Conversely, if the unimaged regions are removed by the developing process, the precursor is negative-working. In each instance, the regions of the imageable layer (i.e., the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Prior to use, printing plate precursors are usually stacked on top of each other during shipping and storage. Adjacent precursors have interposed there between a protective interleaving paper or some other type of interleaf that intimately contacts the surface of the imageable element and prevents the precursors from sticking together. This interleaving paper is removed from the imageable layer prior to imaging.
Imageable elements that are to be imaged by exposure through a photomask typically have a matte layer on the surface to prevent the photomask from sticking to the imageable element during imaging. This matte layer also prevents the interleaving paper from sticking too strongly to the imageable element so that the interleaving paper can be easily released by an automatic interleaving paper releasing machine.
Direct digital imaging of printing plate precursors, which obviates the need for imaging through a photomask, is becoming increasingly important in the printing industry. Because these imageable elements are imaged without a photomask, the imageable layer does not have a matte layer so the interleaving paper would have to be placed directly on the imageable layer. However, when the interleaving paper is placed directly on the imageable layer, the contact area between the imageable layer and the interleaving paper becomes larger and the sheets stick to each other. When the interleaving paper and the imageable layer stick to each other, it becomes difficult to release the interleaving paper with an automatic interleaving paper releasing machine and problems arise, such as paper jamming in the automatic interleaving paper releasing machine. However, when the interleaving paper is omitted, the precursors have a tendency to stick to each other and, thus, can not be easily handled by automatic processing equipment.
Thus, a need exists for imageable elements useful as lithographic printing plate precursors that do not require an interleaving paper yet do not stick to each other when the interleaving paper is omitted so that they can be easily handled by automatic processing equipment.